1. Field of the Invention
The invention relates to detecting gases and, more particularly, to a method for detecting gases, using laser-spectroscopic methods. Specifically, a light source with monochromatic light is used, i.e., a laser or a laser diode, as well as an absorption path or measurement cell, a photodetector, and electronics for controlling the measurement process and for evaluation. The spectral tuning capability of the light source of the laser diode makes it possible to record absorption spectra. The monochromatic form of, for example, a laser diode is necessary to allow the individual vibration/rotation transitions of the gas molecules to be resolved without broadening because of the principle used. In general, a measurement is performed on an absorption measurement path which contains at least one measurement gas or target gas.
2. Description of the Related Art
Carbon monoxide (CO) is, for example, a target gas to be determined. This occurs in the event of incomplete combustion in furnace installations, in engines or in the event of fires. In the building field, a considerable risk with a fatal outcome can occur as a consequence of combustion gases being accidentally misdirected. However, the risk of poisoning can be precluded by monitoring rooms where there is a hazard of toxic carbon dioxide or carbon monoxide concentrations.
For example, if a smoke alarm is equipped with an additional carbon monoxide detector, then this considerably improves the reliability of fire identification. In applications such as this, in which absolute detection reliability is required, human life may depend on the serviceability of corresponding sensors.
Previously known carbon monoxide detection methods already use absorption spectroscopy using tunable laser diodes. The abbreviation for this method is TDLS, Tunable Diode Laser Spectroscopy, with a tunable laser diode or laser.
Such methods have the following characteristic features. An absolute measurement is possible, and a corresponding sensor is resistant to dirt. The spectral measurement over an absorption line of a gas always also includes the background, where the ratio between the minimum transmission of the absorption line and background transmission is proportional to the gas concentration. An absolute change in the transmission, such as due to a window becoming dirty, does not influence the concentration measurement. Normally, light attenuation can rise by several orders of magnitude before the sensor noise increases significantly.
No appliance constants occur in conjunction with the apparatuses that are used. Because of the narrow spectral width of the laser emission, the measured spectra can be compared directly with theoretical spectra without any further instrument function, based simply on molecule constants and physical variables such as pressure, temperature and optical wavelength.
The above-described technique allows a measurement to be performed which is stable in the long term. The measurement principle which is based on fundamental physics, in conjunction with the abovementioned characteristics of the measurement method, allow a gas monitor to be produced which is stable in the long term.
Self-monitoring is performed inherently. Permanent absorption with a reference gas allows continuous monitoring of the serviceability of the sensor.
Malfunctions, such as an escape of the reference gas, interruption in the optical path, a defect in a laser or in a laser diode or in electrical components, can thus be detected immediately.
The maximum possible selectivity is self-evident, because the individual absorption lines of a gas have a narrow bandwidth. As a result, each gas generally has a spectral fingerprint which cannot be confused. The high-resolution measurement with precise detection of individual lines therefore allows extremely selective concentration determination.
Laser spectroscopy using a tunable laser diode generally allows a measurement without any delay, which is highly useful, in particular for exhaust-gas measurements for regulatory purposes.
Furthermore, a series of conventional detection principles are known for verification of carbon dioxide, such as metal oxide sensors, electrochemical cells, color change with synthetic hemoglobin or infrared photometry. By way of example, electrochemical cells play a major role for monitoring room air. However, one disadvantage is that electrochemical cells cannot check their own serviceability. It is therefore necessary after an operating life of several years to ensure that the sensor is still performing a correct measurement, and the serviceability must be regularly tested using a test gas. A carbon monoxide monitor based on TDLS allows self-monitoring.
TDLS-based carbon monoxide detection is used in the professional environment, for example, for monitoring safety at work. Here, not only are the procurement costs significant, but also the running costs, such as for regular checks. Furthermore, freedom from maintenance is an important factor. Fields of use for laser spectroscopy with a tunable laser diode may include fire identification in buildings, in aircraft and in marine vessels, and the control and monitoring of furnace installations, as well as an early indicator of malfunctions in internal combustion engines.
Fundamentally, there are two options for the tuning of tunable laser diodes. Tuning by the temperature of the laser diode, linked to a specific subarea of a spectrum, is relatively slow. If the laser diode is tuned by variation of the operating current, then this is achieved more quickly than in the case of the abovementioned method. In general, the wavelength scale for tuning a laser diode cannot, however, be considered to be linear. That is, the precise structure of the spectrum covered is unknown. Furthermore, a gas to be detected, such as carbon monoxide, is not present in the atmosphere. As a result, the gas is not detected.